This invention relates to the removal of organic impurities from Bayer process liquors. More particularly, it relates to the removal of dissolved sodium oxalate salts from caustic Bayer process liquors.
In the Bayer process, bauxite ore is subjected to digestion with a caustic liquor at elevated temperatures and pressures. The digestion results in a caustic-soluble liquid phase containing substantially all of the caustic-soluble alumina content of the bauxite, together with other caustic-soluble constituents and a caustic-insoluble solid phase generally referred to as "red mud". After separation of the liquid phase (pregnant liquor) from the insoluble solids, the alumina content of the pregnant liquor is usually recovered by seeded crystallization. The liquor depleted in its alumina content is generally referred to as "spent" liquor and this spent liquor is commonly reused for digestion of a fresh charge of bauxite after its caustic concentration is reestablished by evaporation and by addition of "make-up" caustic.
Bauxites, regardless of their geographical origin, always contain organic compounds in varying amounts. These organic compounds are usually referred to as "humates" since they originate from the humus content of the soil present in the bauxite ore. During digestion of the bauxite at the temperatures and pressures employed in both the European and American Bayer processes, these humates oxidize to lower molecular weight organic compounds and in the presence of caustic, produce caustic soluble salts, such as oxalate, acetate, and succinate salts. The oxalate in solution is usually coprecipitated with the alumina content of the pregnant liquor in the form of very fine particles. These fine organic particles form nucleation sites for the alumina hydrate which as a consequence will also precipitate as fine particles. This organic compound can also coat the surfaces of the precipitated alumina hydrate particles and thus prevent agglomeration and/or growth of the hydrate particles. Agglomeration and/or growth is required for the formation of coarse and strong alumina hydrate capable of withstanding the subsequent processing steps, including high temperature calcination and the handling of the calcined alumina.
Humates are not the only source of organics in Bayer liquors. To improve the settling properties of the caustic insoluble phase generated by digestion of bauxite, settling aids or flocculants are added. These settling aids are organic compounds, for example, starch or man-made polymers, and during the course of the Bayer process, some of these also degrade and contaminate the liquor.
Due to the extensive recycling of liquor in the Bayer process, the oxalate level gradually increases to undesirably high levels and unless provisions are made for purification, serious operational problems will arise due to precipitation resulting in product contamination and scaling of process equipment.
Removal of organic impurities from Bayer liquors is a long-recognized necessity and numerous methods were disclosed for accomplishing such a purification. In U.S. Pat. No. 3,372,985 (Roberts et al), an oxalate removal method is described involving the sequential washing of the precipitated alumina hydrate. In the first stage, adhered caustic is removed from the surface of the hydrate by a cold water wash; in the second stage, hot water is utilized which dissolves sodium oxalate without hydrate losses. From the sodium oxalate solution, oxalate is removed by addition of lime which will form an essentially insoluble calcium oxalate solid. U.S. Pat. No. 3,649,185 (Sato et al) employs temperature control to reduce the solubility of sodium oxalate and precipitate oxalate crystals from a Bayer process stream rich in oxalate content and having a low dissolved alumina content. The removal efficiency varies as a function of temperature, caustic concentration, and oxalate content. Seeded crystallization is utilized in U.S. Pat. No. 3,899,571 (Yamada et al) to remove oxalate impurities from Bayer liquors. The process of this patent involves the addition of sodium oxalate crystals to a sodium aluminate solution contaminated with oxalate. The quantity of oxalate seed added is selected to be sufficient for shifting the equilibrium solubility of the dissolved oxalate impurity content resulting in precipitation of a portion of the dissolved oxalate. The precipitated oxalate is washed and a portion of the washed oxalate is recycled as seed for the treatment of an oxalate-contaminated sodium aluminate solution.
Removal of oxalate from a concentrated Bayer process stream by the use of a packed column is shown in U.S. Pat. No. 4,038,039 (Carruthers et al). Spent liquor is concentrated by evaporation and the concentrated liquor is sprayed on the top of a column packed, for example, with steel turnings. The concentrated liquor is supersaturated with sodium oxalate and as this liquor contacts the packing, precipitation of oxalate will take place. Purified liquor is recovered from the column and the oxalate precipitate is removed from the packing surfaces by hot water wash.
Harmful organic compounds are removed from Bayer process liquors by treating the contaminated liquor with a magnesium compound in accordance with the teachings of U.S. Pat. No. 4,046,855 (Schepers et al). The magnesium compound forms a precipitate when added to the impure liquor and the precipitate will contain a portion of the organic impurities which can be removed from the liquor, together with the precipitate. U.S. Pat. No. 4,215,094 (Inao et al) employs an oxidative process for the removal of organics from Bayer liquors. Contaminated Bayer liquor is contacted at elevated temperatures and pressures with oxygen in the presence of a copper catalyst. Oxidation degrades the organics to carbon dioxide and thus decreases the organic content of the treated liquor.
In U.S. Pat. No. 4,263,261 (Yamada et al) sodium oxalate impurities are removed from Bayer process liquors by seeded crystallization. Sodium oxalate crystals are added with vanadium and/or phosphorus-containing salts and the resulting precipitate is reused for renewed impurity removal by crystallization. The oxalate level of impure Bayer liquors can, according to U.S. Pat. No. 4,335,082, be controlled by causticizing the liquor with lime, followed by evaporation. Evaporation increases the caustic concentration of the liquor to 200-400 g/l, at this concentration solids will precipitate containing a portion of the organic impurities. The purified liquor is recycled to the Bayer process while the precipitate is discarded.
From the extent of the prior art discussed above, it can be seen that the presence of oxalate in Bayer liquors is a serious problem and the numerous processes advanced by the prior art indicate the continued need for a simple but effective and economical oxalate removal process.
It has now been found that sodium oxalate impurity can be removed in an efficient and economical manner from Bayer process streams by gradually increasing the caustic concentration of the liquor, thus providing for a staged removal of the oxalate content by precipitation.